Conventionally, a generally known diesel engine is provided with a fuel injection device, which injects the high-pressure fuel. Such a fuel injection device includes, for example, an injector mounted to each cylinder of the engine. In the fuel injection device, a low-pressure fuel pump draws fuel from a fuel tank and preliminary pressurizes the drawn fuel. The low-pressure fuel pump feeds the pressurized fuel as low-pressure fuel to a high-pressure pump. The high-pressure pump further pressurizes the low-pressure fuel to be high-pressure fuel at fuel injection pressure.
For example, JP-A-2006-46323 proposes a common rail type fuel injection device having a piezo injector, which is excellent in response. The piezo injector includes a hydraulic pressure transmission mechanism, which transmits displacement of a piezo stack via hydraulic pressure. The hydraulic pressure transmission mechanism actuates a control valve for controlling pressure in a control chamber at the side of a nozzle needle. Thus, the hydraulic pressure transmission mechanism controls the nozzle needle to open and close nozzle holes so as to therethrough inject high-pressure fuel. The hydraulic pressure transmission mechanism includes a first piston, which is located at the side of the piezo stack, a second piston, which is located at the side of the control valve, and a hydraulic chamber provided between the first and second pistons. The hydraulic chamber and the first and second pistons are located in series in a cylinder. In the present structure, the hydraulic chamber containing fuel is interposed between the cylinders. Therefore, thermal expansion caused differently in components can be absorbed by supplying fuel into the hydraulic chamber and leaking fuel from the hydraulic chamber. Thus, pressure transmission and fuel injection performance can be maintained.
The piezo stack extends when being energized, and thereby increase pressure in the hydraulic chamber so as to transmit driving force to the control valve for fuel injection. At the time of stop of the fuel injection, the piezo stack is de-energized, and the driving force of the piezo stack is eliminated. Therefore, the hydraulic chamber is reduced or increased in pressure in response to the amount of fuel leak, and thereby fuel flows into the hydraulic chamber. That is, the hydraulic chamber is charged with fuel from the back pressure side of the injector.
In the present conventional art, the back pressure side of the injector connects with the leak line through which fuel is returned to the fuel tank. Therefore, fuel is leaked from the back pressure side of the injector or fuel flows into the back pressure side of the injector in conjunction with fuel injection of the injector, and thereby injection performance is supposed to be maintained. However, according to study of the present inventors, startability may be impaired in exchange for injection performance. The reason will be described as follows. So as to secure an injection performance, it is conceived to provide a check valve so as to maintain fuel in the leak line at positive pressure. By providing the check valve, fuel charge into the hydraulic chamber can be ensured, and fuel at the side of the back pressure of the injector can be maintained at positive pressure. In addition, so as to maintain the back pressure side of the injector at positive pressure regardless of engine starting or normal operation, low-pressure fuel needs to be fed to the back pressure side of the injector by communicating the feed line of the outlet of the low-pressure fuel pump with the back pressure side of the injector.
However, in the present circuit, in which the check valve maintains the back pressure side of the injector at positive pressure, the feed line may be regularly communicated with the leak line, i.e., the outlet of the low-pressure fuel pump may be regularly communicated with the fuel tank when the check valve causes a malfunction and regularly communicates therethrough. If the check valve causes a malfunction and regularly communicates therethrough, the low-pressure fuel pump cannot properly pressurize fuel. As a result, startability of the engine may be impaired, and engine operation cannot be maintained. Furthermore, in the present circuit, in which the feed line of the outlet of the low-pressure fuel pump is connected to the back pressure side of the injector, another check valve needs to be provided to the feed line so as to enable restarting in the case where fuel entrains air. Therefore, the two check valves are needed in the present circuit, and consequently the entire system of the circuit increases in cost.